There is known a drive system for a vehicle, which includes a differential mechanism operable to distribute an output of an engine to a first electric motor and its output shaft, and a second electric motor disposed between the output shaft of the differential mechanism and drive wheels of the vehicle. Patent Document 1 discloses an example of such a vehicular drive system, which is a hybrid vehicle drive system. In this hybrid vehicle drive system, the differential mechanism is constituted by a planetary gear set, for example, and a major portion of a drive force of the engine is mechanically transmitted to the drive wheels through the differential function of the differential mechanism, while the rest of the drive force is electrically transmitted from the first electric motor to the second electric motor, through an electric path therebetween, so that the vehicular drive system functions as a transmission the speed ratio of which is continuously variable, for example, as an electrically controlled continuously variable transmission, thereby making it possible to drive the vehicle under the control of a control apparatus, with the engine kept in an optimum operating state with an improved fuel economy.    Patent Document 1: JP-2003-301731 A    Patent Document 2: JP-7-285360 A    Patent Document 3: JP-60-241570
Generally, a continuously variable transmission is known as a transmission which permits an improved fuel economy of the vehicle, while on the other hand a gear type transmission such as a step-variable automatic transmission is known as a transmission having a high power transmitting efficiency. However, there is not available any power transmitting mechanism having the advantages of those two types of transmission. For example, the hybrid vehicle drive system disclosed in the above-identified Patent Document 1 includes the electric path for transmitting an electric energy from the first electric motor to the second electric motor, namely, a power transmitting path for transmitting a portion of the vehicle drive force as an electric energy, so that the first electric motor is required to be large-sized to meet a need for an increased output of the engine, and the second electric motor driven by the electric energy generated by the first electric motor is also required to be accordingly large-sized, whereby the overall size of the hybrid vehicle drive system tends to be large-sized. It is also noted that a portion of the output of the engine is once converted into an electric energy which is subsequently converted into a mechanical energy to be transmitted to the drive wheels, whereby the fuel economy of the vehicle may possibly be deteriorated under some running condition of the vehicle, for instance, during a high-speed running of the vehicle. Where the above-described differential mechanism is a transmission the speed ratio of which is electrically variable, for example, a continuously variable transmission so-called an “electric CVT”, the vehicular drive system suffers from a similar problem.
The present invention was made in view of the background art described above. It is therefore an object of this invention to provide a control apparatus for a vehicular drive system including a differential mechanism operable to perform a differential function for distributing an output of an engine to a first electric motor and its output shaft, and a second electric motor disposed in a power transmitting path between the differential mechanism and a drive wheel of a vehicle, the control apparatus permitting reduction of the required size or an improvement of the fuel economy of the vehicular drive system.